A technique for improving the performance of an integrated circuit formed on a substrate, such as a memory device, is to provide a separate potential to the substrate instead of coupling the substrate to a predetermined potential, usually the 5 volt power supply or a ground reference potential, as appropriate. The supply potential may be either a positive 5 volts or a negative 5 volts. The value of the potential may be more negative than either the ground reference potential or the negative 5 volts or more positive than the positive 5 volts. Typically, a p-type substrate layer or well is pumped to a more negative potential by a substrate pump.
The substrate potential is typically generated with an on-chip circuit containing a substrate charge pump used to pump the p-type substrate to a more negative potential. When the substrate layer or well potential changes from a correct value due to leakage or a change in the operating condition of the integrated circuit, a regulator detects the change and provides an output signal to activate a charge pump. In turn, the charge pump pumps charge into the substrate layer until the substrate layer or well potential returns to the desired or regulated value. The regulator then provides an output signal to deactivate the charge pump.
A high voltage pump is typically used to pump positive charge into a high voltage bus, such as a wordline driver.
A voltage generator circuit includes the regulator for sensing the potential of the substrate and for providing an output that is coupled to an inverter. The inverter provides a control signal directly or indirectly to the charge pump. Normally a hysteresis circuit couples the control signal to the charge pump input terminal. The charge pump activates and provides an output that is desired to be regulated. The hysteresis circuit eliminates erratic switching by preventing the charge pump from constantly turning on and off.
Voltage generator circuits draw a significant current that flows directly out of the p-type substrate through the sense element. This current directly and indirectly increases the power requirements of the voltage generator circuit; directly because of the power consumption due to the current flowing through circuit components and indirectly due to the added current requirements to compensate for the current flowing out of the substrate through the sense element. This power consumption is only significant during standby; and since most of the standby current is generated in the regulator circuit, reductions of the standby current in the regulator portion of the memory device significantly enhance the operation of the memory device. Normally, in the case of a negatively charged p-type substrate layer or well, the sense element current further raises the substrate potential. Therefore, the charge pump must be activated more frequently to maintain a nominal substrate potential.
A regulator circuit 5 is shown in detail in the voltage generator circuit 6 depicted in FIG. 1. The regulator circuit 5 comprises the sense element 7 and an inverter 8. The sense element 7 comprises two metal oxide semiconductor field-effect transistors (MOSFETs) connected as a diode series 10, although more or less diodes may be used depending on the desired value of V.sub.BB. The diode series 10 is connected directly to the substrate layer at a sense node 15 and is connected to V.sub.CC 16 through a load element 20. The MOSFET diode series 10 and the load element 20 are connected at an intermediate node 25. The diode series 10 and the load element 20 are known as a level shifting circuit since the potential at the intermediate node 25 is dependent on the potential drop across the diodes series 10. As the sense node potential V.sub.BB increases due to circuit leakages, the intermediate node potential increases. Eventually, the intermediate node potential will be high enough to gate the inverter 8. The inverter 30 comprises an input switching n-type MOSFET (NMOSFET) 35 serially connected to a load p-type MOSFET (PMOSFET) 40 at the inverter output 45. Thus, a shift in the potential at the intermediate node from a low level to a high level causes the inverter to activate a charge pump 50 through the hysteresis circuit 55. The charge pump output is connected to the sense node 15, and the activated charge pump reduces the potential of V.sub.BB. This reduced potential is also reflected at the intermediate node 25 and the inverter output signal at 45 switches states and the charge pump is turned off.
A significant current draw in the circuit of FIG. 1 is the current consumed in the inverter 8. Since the gate of transistor 40 is tied to ground, whenever transistor 35 is gated there is a high current consumption through the inverter 8. This current effected in the inverter due to a continually gated device is called bleeder current.
FIG. 2 is a timing diagram based on a computer simulation of the circuit of FIG. 1 and relating the sense node potential, V.sub.BB 60, to the charge pump input potential, ENV.sub.BB 65. The sense element provides an output signal, V.sub.1 70, at the intermediate node to the inverter input. The inverter output provides a control signal, V.sub.2 75. The control signal, V.sub.2 75, activates the charge pump through the hysteresis circuit. By analyzing ENV.sub.BB 65 it can be seen that the charge pump is activated every 2.8 microseconds when ENV.sub.BB goes high. This charge pump activation frequency is based on the circuit having a large load. The frequency increases with a decrease in load.
Since the charge pump is typically 25-35% efficient, an additional 1 microamp (.mu.A) of current flowing in the sense element translates to an additional 3-4 .mu.A of current that must be consumed by the charge pump. Typically, 5 .mu.A of current is required by the sense element to maintain a reasonably short delay time to respond to changes in the substrate potential. Thus, a total of 20-25 .mu.A of additional current is consumed by the voltage generator circuit.
One simple way to reduce the current requirements of the voltage generator circuit is to decrease the current flowing through the sense element by increasing the value of the load element. Similarly the current in the inverter can be decreased by increasing the resistance of transistor 35, thereby reducing the bleeder current. Such a decrease in current in the inverter, however, produces a corresponding undesirable increase in the delay time in response to changes in the substrate potential. Thus, the accuracy of the regulated substrate potential decreases resulting in decreased performance and, possibly, decreased immunity to latch-up of the integrated circuit.
What is desired is a voltage generator circuit for regulating the potential of a substrate on an integrated circuit having a low current requirement yet maintaining a reasonable delay time in responding to changes in the substrate potential.